1. Field of the Invention
This invention pertains to friction welding and more particularly to a portable inertia friction-welding machine.
2. Background of the Invention
Friction welding is a solid-state process in which workpieces are placed under compressive force with each other. The contact surfaces (joint or faying surfaces) are moved with respect to each other, typically by rotation, to produce sufficient heating to weld the workpieces together. Various types of friction welding are detailed in the AWS Welding Handbook, 8th Ed., Vol. 2, American Welding Society, 1991.
Two types of friction welding are in common use: 1) direct-drive friction welding and 2) inertia friction welding. In direct-drive friction welding, one of the workpieces is loaded into a rotating spindle and brought into contact with a second workpiece under a low compressive force referred to as a first friction force or preheat force. In some cases, the first friction force may not be used. Frictional heating is continued for a preset time or displacement distance, i.e., the distance the two workpieces move toward each other due to the first compressive force. Typically a higher compressive force, termed the second friction force or welding force is then applied causing plasticized metal to be extruded from the joint (contacting surfaces of the two workpieces). Again the second compressive force may be applied for a preset time or distance. The rotating spindle is then brought to a stop by means of a braking system such as a caliper and/or by means of an electric brake in the spindle drive motor. A third compression force, typically referred to as the upset force, is applied to consolidate the joint. The upset force may be applied while the spindle is braking or after it stops. If the upset force is applied while braking, the microstructure of the joint reflects a rotational-type of forging. If the upset force is applied after the spindle has stopped, the microstructure of the joint reflects an axial-type of forging.
In inertia friction welding, the frictional heating is provided by stored rotational kinetic energy in the form of one or more flywheels mounted on the rotating spindle. The inertia of the system is changed by either adding or removing flywheels from the spindle or changing the spindle rotational speed or both. To begin the welding process, the two workpieces are loaded into the welding machine, typically one workpiece is secured to the spindle and the other workpiece secured to the tailstock of the machine. The spindle is then accelerated to a predetermined velocity, i.e., rotational speed typically expressed in revolutions per minute (rpm)), and the spindle drive means is disengaged allowing the spindle, with attached flywheel(s), and the workpiece mounted in the spindle to coast. The rotating workpiece is then brought into contact with the stationary workpiece with an applied compressive force to begin the frictional heating of the two workpieces. The flywheel comes to a stop as its kinetic energy is transitioned into thermal energy (frictional heating) at the interface of the two workpieces. In some cases, as the flywheel slows down or after it has stopped, the compressive force is increased so as to apply an upset force to complete the joint. Additional details as to inertia and direct-drive friction welding can be found in T. Stotler, xe2x80x9cProcedure Development and Practice Considerations for Inertia and Direct-Drive Friction Weldingxe2x80x9d in ASM Handbook, Volume 6, ASM International, United States, 1993, pp. 888-891.
As noted above, friction welding has largely focused on direct-drive and inertia welders. Both of these welders have been extremely large units securely mounted to the shop floor and often weigh a ton or more. U.S. Pat. Application Pub No. US 2002/0036225 Al illustrates a friction-welding machine while U.S. Pat. Nos. 3,235,162; 3,234,644; 3,571,905; 3,591,068; and 4,030,658 illustrate inertia welding machines. In both the direct-drive and inertia machines, the tailstock-mounted workpiece has been used to apply the compressive forces by axially moving the tailstock-mounted workpiece toward the rotating second workpiece mounted in a rotating chuck. That is, the rotating chuck (and a first workpiece) was mounted to be axially stationary; the second workpiece was secured in the tailstock piece to be rotationally stationary but moveable in an axial direction in order to apply the requisite compressive force. Typically both the rotating chuck and the tailstock were mounted on a base with the tailstock drawn toward the rotating chuck by mechanical means. Some machines use a fixed tailstock in which compressive forces are achieved by moving the rotating unit by means of a carriage and piston assembly.
One of the first attempts at producing a portable machine is found in U.S. Pat. No. 3,185,368. Although various improvements were made to make the machine as light as possible, it remained a massive machine that was truck mounted for use in the field. Although there are some portable friction welders on the market, these machines have difficulty welding large diameter parts (i.e., greater than about xc2xe-in.) or welding materials that need short welding times such as aluminum, due to the high torque requirements. These machines are direct-drive friction welders to avoid the need for large and heavy flywheels although, occasionally a small amount of additional weight is occasionally used to overcome the initial torque peak and motor stalling that occurs when the workpieces first touch. Even in these cases, the only way to overcome the low-torque capabilities is to reduce the initial compressive forces significantly. However, this dramatically increases the weld time and, unfortunately, such long weld times often have a detrimental affect on the mechanical properties of the resulting weld of the workpiece materials. At best, these machines can only friction weld small studs or appurtenances (less than about xc2xd inch in diameter) to plates or pipe with a resulting low pull-off strength, typically less than 30,000 lbs.
To achieve the required torque and spindle speed for a direct-drive unit for large appurtenances with a 30,000 lb pull off strength would require at least a ten horsepower electric motor in combination with a large gear box. This results in a machine that is either too large and heavy to carry and position, i.e., is no longer portable, or too large to obtain access to the welding area, e.g., the inside of a vehicle. To even consider the use of inertia welders with their large and heavy flywheels for welding appurtenances greater than xc2xd or xc2xe inch diameter flies in the very face of portability.
In addition to the inability of current portable direct-drive friction welders to weld large diameter appurtenances, i.e., appurtenances having a minimum pull-off strength of 30,000 lb, there are additional problems in using such units, especially when attempting to weld internally threaded, large diameter appurtenances. First of all, plasticized metal formed during the welding process extrudes into the internal threads at the base of the appurtenance and blocks the effective use (maximum engagement) of the internal threads. Second, the plasticized metal also extrudes on the external surface of the appurtenance and can interfere with the attachment of parts to the plate using the appurtenance. That is, the part cannot be screwed into the appurtenance and brought flush with the plate to which the appurtenance is joined. The extruded metal (flash) can also promote crevice corrosion when the resulting assembly is exposed to hostile environments such as seawater or corrosive chemicals. Third, it is difficult to grasp an internally threaded appurtenance in the welder to effectively produce both rotational and compressive axial forces. This can result in rotational slippage and, as a result, there may be insufficient heating to produce a satisfactory weld. It can also result in axial slippage that can result in both insufficient heating and compressive (forging) force to achieve a satisfactory weld. Fourth, it has been found that there can be more than a fifty percent loss of weld strength when no steps are taken to prepare the plate surface to which the appurtenance is attached.
However, there are many applications (e.g., heavy-duty construction and military vehicles, planes, sea vessels and other units) in which it is desirable to mount various large fasteners, mounting projections, and so forth, i.e., appurtenances, to panels (e.g., floors, walls, ceilings). Unfortunately and due to the various factors noted above, it has not been possible to use friction welding machines (either direct drive or inertia) for the joining of heavy-duty (large) appurtenances. As such, other more portable welding techniques such as gas metal arc welding (GMAW) have been used to weld these appurtenances to the requisite panels. Unfortunately, GMAW is a relatively time-consuming method that requires a high degree of welder skill due to the weld joint position and location within a particular unit. A typical time for welding a single appurtenance is about thirty minutes. Further, some mechanical loss also occurs when using GMAW. Given that certain units may require up to a thousand appurtenances, it becomes apparent that a faster and easier method of appurtenance attachment is a critical need, especially in current times when the development of rapid methods for the assembly of all-terrain vehicles, planes, and vessels is essential to national security.
Accordingly, it is an object of the present invention to provide a portable friction-welding machine for producing welds with a pull off strength greater than 30,000 lbs.
It is another object of the present invention to provide a portable friction-welding machine for welding appurtenances with a diameter greater than about xc2xd inch in diameter and preferably greater than xc2xe inch diameter.
It is another object of the present invention to provide a portable inertia welder.
It is an object of the present invention to provide an internally threaded appurtenance that avoids obstruction of the internal treads by flash from the welding process.
It is another object of the present invention to attach the welding machine to the surface to which the appurtenance is to be welded in a stable manner that allows for complete transfer of rotational and compressive forces to the welding process.
It is a further object of the present invention to provide a method of preparing the plate to which the appurtenance is attached that maximizes weld strength.
These objectives are met with the present invention of an inertia welder that comprises a welder housing and a drive shaft that is capable of movement in the direction of its longitudinal axis as well as of rotational movement within its mountings to the housing. A motor, typically hydraulic or pneumatic, is mounted to the drive shaft and drives the drive shaft in rotation. A flywheel is mounted on the drive shaft so that it rotates with the drive shaft. A hydraulic housing with a cylindrical bore is mounted to the welder housing in a fixed position and contains a moveable piston within the cylindrical bore. One end of the piston and a first portion of the hydraulic housing form a first hydraulic chamber; the opposite end of the piston and a second portion of the hydraulic housing opposite the first portion forms a second hydraulic chamber. A first opening in the first portion of the hydraulic housing allows for the passage of hydraulic fluid into and out of the first hydraulic chamber by means of a first hydraulic line. Similarly, a second opening in the second portion of the hydraulic housing allows for passage of hydraulic fluid into and out of the second hydraulic chamber by means of a second hydraulic line.
The drive shaft rotatably passes through the hydraulic housing and through the piston to which it is axially secured. That is, the drive shaft rotates freely within the hydraulic housing and also moves axially (along it axis) within the hydraulic housing. Because the drive shaft is secured axially to the piston, the piston and the drive shaft move together in the direction of the longitudinal axis of the drive shaft. Preferably the drive shaft rotates within the piston but it is possible that the piston may also be rotationally fixed to the drive shaft in which case, the piston rotates within the hydraulic housing. When it is desired to move the piston longitudinally, hydraulic fluid is pumped into the first hydraulic chamber and removed from the second chamber. When it is desired to move the piston longitudinally in the opposite direction, hydraulic fluid is pumped into the second chamber and removed from the first chamber.
A chuck is mounted at the end of the drive shaft opposite the motor and secures and delivers both rotational and longitudinal forces from the drive shaft to a first workpiece sufficient to weld the first workpiece to a second workpiece.
One of the features of the present invention is that both the rotational force and the longitudinal force are delivered through the same shaft which has the advantage of reducing considerably the weight of the machine by eliminating bulky tailstock assemblies for securing and longitudinally moving the second workpiece. This is achieved by securing the welder housing to the second workpiece thereby positioning the second workpiece in a fixed position with respect to the welder. This completely eliminates the tailstock and allows the drive shaft to deliver both the rotational and longitudinal forces for the welding process. The welder housing can be secured to the second workpiece in a variety of ways including the use of mechanical clamps, vacuum clamps, and fasteners.
Since the motor is attached at one end of the drive shaft, it must be secured to the housing in such a manner as to prevent its rotation while at the same time it must be able to move translationally with the drive shaft. To this end, the present invention uses one or more guide rods that are secured to the housing and which pass through the motor housing in a direction parallel to the axis of the drive shaft and prevent rotation of the motor. However, the motor housing is free to move translationally along the guide rods thus allowing both the motor and drive shaft to move longitudinally in a direction along the longitudinal axis of the drive shaft.
Often the first workpiece, i.e., the workpiece rotated and axially driven by the drive shaft, is a circular appurtenance with a bottom portion and a top portion. For optimal performance of the inertia welder, it is desirable to have the drive shaft effectively and efficiently transfer both the rotation and linear axial forces to the appurtenance. To this end, the present invention features circular appurtenances having at least two opposing shoulders formed in the top portion which have the advantage of allowing for quick securement within the flats of the chuck and which have the further advantage of reliably transferring the rotational and longitudinal forces from the drive shaft.
Another feature of the present invention is the formation of a flash trap in the bottom interior portion of the circular appurtenance. The flash trap is formed by boring a hole in the bottom portion of the circular appurtenance that has a common axis with the circular appurtenance. The flash trap has the advantage of allowing for the accumulation of workpiece material (flash) as it extrudes from the interface of the two workpieces as they are welded together thereby preventing entry of the extruded material into a central bore within the appurtenance. Thus extruded material can be especially problematic when the bore is a threaded bore designed for the acceptance of a threaded member after the weld is complete.
Although there have been claims that cleaning of the workpiece is not necessary when a friction welder is used, it has been found that workpiece cleaning can impact significantly the strength of the weld formed using friction welding. To this end another feature of the present invention is the cleaning of the workpiece, especially workpieces formed from aluminum and aluminum alloys. Abrasive sanding and milling have been found to have the advantage of improving the weld strength by up to fifty percent. Contrary wise, wire brushing has been found to have little effect since it appears that the brushing technique merely grinds the impurities and oxides into the surface.
Finally it is to be realized that as the plasticized workpiece material extrudes from the interface between the two workpieces during the welding process, it forms a bead or curl of flash. For a circular appurtenance, the flash is formed around the circular perimeter of the appurtenance. Such flash can be especially troublesome when the appurtenance is used to anchor a fitting that is screwed into a threaded bore within the appurtenance. Often the flash prevents flush engagement of the fitting with the plate (second workpiece) to which the appurtenance is attached. To this end the present invention features a flash cutter that has at least two opposing flat sides that engage the flats of the chuck which is driven rotationally and longitudinally by the drive shaft. This has the advantage of allowing the flash to be quickly removed after the welding operation without having to reposition the inertia welder and avoids completely the use of other methods of flash removal.
The foregoing and other objects, features and advantages of the invention will become apparent from the following disclosure in which one or more preferred embodiments of the invention are described in detail and illustrated in the accompanying drawings. It is contemplated that variations in procedures, structural features and arrangement of parts may appear to a person skilled in the art without departing from the scope of or sacrificing any of the advantages of the invention.